Astronomers can image the environments of nearby stars on scales comparable to our own solar system. New classes of astrophysical objects have been discovered including circumstellar debris disks, brown dwarfs, and super-Jupiter mass planets. These discoveries have galvanized intense public interest in science and technology and have led to profound new insights into the formation and evolution of planetary systems such as our own. Among the key technologies that have enabled this are adaptive optics (AO) and coronagraphy, both of which deliver the high image contrast for the discovery and characterization of faint stellar companions and circumstellar disks in the solar neighborhood. The highest order AO system available to the astronomical community in the United States is the Air Force Advanced Electro-Optical System (AEOS), located at the Air Force Research Laboratory's (AFRL) Maui Optical Site (AMOS) on Maui.
AEOS is a 3.67-meter telescope owned by the Department of Defense. The 75-ton AEOS telescope points and tracks very accurately, yet is fast enough to track both low-Earth satellites and missiles. AEOS can be used simultaneously by many groups or institutions because its light can be channeled through a series of mirrors to seven independent coudé rooms below the telescope. Employing sophisticated sensors that include an adaptive optics system, radiometer, spectrograph, and long-wave infrared imager, the telescope tracks man-made objects in deep space and performs space object identification data collection. This is part of Strategic Command's Space Surveillance effort.
AEOS is equipped with an adaptive optics system, the heart of which is a 941-actuator deformable mirror that can change its shape to remove the atmosphere's distorting effects. Using this, scientists can gather near diffraction-limited images of space objects.
To block light from a bright star in order to detect faint, nearby objects, a coronagraph is used. The optical elements in the coronagraph are used to create an image of the pupil plane, essentially an image of the telescope's primary mirror, and any support structures which enter the telescope's beam path. These elements scatter a substantial amount of light and degrade the contrast of the final image produced by the telescope. For example, the bright spikes present around stars in many astronomical images are a result of diffraction around the secondary mirror support spider. Faint objects located on or near these spikes are very difficult to detect. To suppress this scattered light, an occulting spot and a Lyot stop (e.g., Lyot mask) are used. The occulting spot is placed at the focal plane of the telescope and at a location coincident with the image of a bright source of light (such as a star or bright satellite). This removes a large fraction of the light from the bright source, while having virtually no effect on other light sources elsewhere in the image. This also reduces subsequent scattering due to light striking any optical elements further down in the optical path. Without the occulting spot, light from the star would saturate the detection device, and the optical elements would fill the background with scattered light, as well as produce spurious reflections. The Lyot stop is used to block the diffracted light from the bright source due to the telescope support structures and the occulting spot, so that stars imaged with a coronagraph do not have diffraction spikes. The diffraction by the occulting spot leaves a ring of residual light from the bright source in a subsequent image of the pupil plane. The Lyot stop blocks this bright ring, substantially reducing the broad halo of light observed in telescopic images of bright sources. The removal of the diffraction spikes and the halo allows for imaging of faint sources near the star. However, it has not been previously possible to observe a planet which is orbiting a star (other than the sun) with a coronagraph. In some cases of observing the planets, the contrast can be larger than 1 part in billions. It has also not been possible to observe microsatellites placed near larger satellites. These microsatellites can be used to destroy larger satellites.